High mobility multilayered heterojunction devices employing modulated doping

Abstract

The mobility of a relatively narrow bandgap semiconductor material can be significantly enhanced by incorporating it into a multilayered structure (10) comprising a first plurality of relatively narrow bandgap layers (12) of the material and a second plurality of wider bandgap semiconductor layers (14) interleaved with and contiguous with the first plurality. The wide bandgap and narrow bandgap layers are substantially lattice-matched to one another, and the wide bandgap layers are doped such that the impurity concentration-thickness product therein is greater than the same product in the narrow bandgap layers. The fabrication of the structure by MBE to enhance the mobility of GaAs is specifically described. In this case, the narrow bandgap layers (12) comprise GaAs and are unintentionally doped to about 10 14 /cm 3 , whereas the wide bandgap layers (14) comprise AlGaAs doped n-type to about 10 16 to 10 18 /cm 3 . The incorporation of this structure in an FET is also described.

We claim: 
     
       1. A high mobility semiconductor device comprising: a first plurality of narrow bandgap semiconductor layers, and   a second plurality of wide bandgap semiconductor layers interleaved with and contiguous with said first plurality,   said wide bandgap and narrow bandgap layers being substantially lattice-matched to one another so as to form substantially defect-free heterojunctions at the interfaces between said layers,   said layers having a conduction or valence band step of sufficient magnitude to confine carriers to said narrow bandgap layers, and characterized in that   said layers are adapted such that the impurity-concentration-thickness product of said wide bandgap layers exceeds the impurity-concentration-thickness product of said narrow bandgap layers.   
     
     
       2. The device of claim 1 wherein said wide bandgap layers are doped only in a central portion thereof so as to leave thin, undoped buffer zones adjacent said heterojunctions. 
     
     
       3. The device of claim 2 wherein said buffer zones are about 10-60 Angstroms thick. 
     
     
       4. The device of claim 1 or 2 wherein said narrow bandgap layers are undoped. 
     
     
       5. The device of claim 4 wherein said narrow bandgap layers have an impurity concentration level of about 10 14  /cm 3  and said wide bandgap layers have an impurity concentration of at least 10 16  /cm 3 . 
     
     
       6. The device of claim 5 wherein said narrow bandgap layers comprise Al y  Ga 1-y  As and said wide bandgap layers comprise n-type Al x  Ga 1-x  As, 0≦y, x-y≳0.02. 
     
     
       7. The device of claim 6 wherein said layers have a thickness in the range of 100 to 400 Angstroms. 
     
     
       8. The device of claim 4 wherein said Al x  Ga 1-x  As layers are doped n-type in the range of about 10 16  to 10 18  /cm 3  approximately, and including means for cooling said device in order to increase its mobility as compared to that at room temperature. 
     
     
       9. The device of claim 8 wherein said cooling means includes cryogenic apparatus for cooling said device below approximately 50° K. so as to increase the mobility of said device to at least 16,000 cm 2  V -1  sec -1  approximately. 
     
     
       10. A field effect transistor comprising a semi-insulating semiconductor substrate,   an n-type device according to claim 1 or 2 fabricated on said substrate,   spaced apart n-type zones extending through said device to said substrate, thereby forming source and drain zones,   a portion of said device extending between said source and drain zones forming the channel of said transistor,   source and drain electrodes formed on said source and drain zones, respectively, and   a Schottky barrier gate electrode formed on said portion between said source and drain electrodes.   
     
     
       11. The transistor of claim 10 wherein said narrow bandgap layers comprise Al y  Ga 1-y  As and said wide bandgap layers comprise n-type Al x  Ga 1-x  As, 0≦y, x-y≳0.02.